1. Field of the Technology
The technology relates to a toner supply device, image forming apparatus and toner shortage detecting method, in particular relating to an image forming apparatus that performs image formation with toner as well as a toner supply device and toner shortage detecting method for use in the image forming apparatus.
2. Description of the Prior Art
Recently, there have been increased demands for image forming apparatuses capable of high-speed operations, and as the number of printing (per unit time) increases the speed of the paper to be conveyed has been also enhanced. For example, conventionally the processing ability of an image forming apparatus with not lower than 60 sheets per minute (A4 short-edge feed) was previously regarded as a high-speed machines, but nowadays, the situation has changed and the machines having a processing speed of 80 sheets per minute or greater should be regarded as high-speed ones, and further, machines having a speed of 100 sheets per minute are being developed.
Since a large amount of toner is consumed in such image forming apparatuses, most of the developing units use a technology for keeping the toner concentration in the developing hopper constant and avoiding indication of “toner empty” from occurring when a large volume of printing has been performed. That is, the developing unit includes a plurality of toner containers arranged, and the toner supplied from each toner container is not directly fed to the developing hopper but is once collected in a toner feed device that functions as a “relay box”, then is fed into the developing hopper as the toner concentration therein becomes lower (see patent document 1: Japanese Patent Application Laid-open Hei 03-220577).
However, this toner supplying method entails a problem. That is, when toner in one of the toner containers runs out after the toner is supplied from that toner container into the toner feed device, its toner empty is detected and a “toner empty” indication is given on the display portion of the image forming apparatus. However, when this indication of toner empty is given in the same manner as used to be, the message for replacement of the empty toner container is prone to be unnoticed or forgotten because toner supply is continued from the other toner containers while the high-speed printing in progress continues as stated above.